Wind speed fluctuates over time. Some wind turbines are not able to track these fluctuations and rotate only at a single speed (frequency). A way of operating at a fixed speed despite variations in wind speed is to use a synchronous generator or a directly connected induction (asynchronous) generator.
Since the maximum power available from the wind is a function of wind speed, and since the power captured by a propeller of a wind turbine is a function by rotor speed and wind speed, fixed speed wind turbines fail to recover this maximum power. Fixed speed turbines also suffer from noise, reliability problems and high stresses on the utility grid. Furthermore the lagging power factor of a grid-connected asynchronous generator demands a large capacitor battery to compensate for the lagging power factor. Accordingly, variable speed implementations have been proposed to recover the maximum power of the wind and better address these other problems of fixed speed turbines. Examples of these variable speed wind turbines are described in U.S. Pat. Nos. 5,083,039 and 5,225,712, and PCT Application US99/07996, each of which is incorporated by reference herein in its entirety.
A variable speed wind turbine 100 is also shown in FIG. 1. One or more wind turbine blades (not shown) drives rotor shaft 111 of asynchronous doubly-fed induction generator 110. Turbine 100 supplies power from rotor 112 and stator 113 of generator 110 when shaft 111 is rotating above synchronous speed. At speeds above synchronous speed, excitation power may be supplied to rotor 112 from rotor inverter 151 in order to achieve unity power factor at the stator side. At shaft speeds lower than synchronous speed, power is supplied from stator 113 and slip power along with the excitation power is supplied to rotor 112 from rotor inverter 151.
To supply power from stator 113, Y/Δ-contacter 130 shifts the three stator windings selectively into a Y-connection or a Δ-connection. FIG. 1A shows the Y-connection and FIG. 1B shows the Δ-connection of the stator windings. The purpose of Y/Δ-switch 130 is to achieve a higher operational speed range and to reduce iron losses in the stator. Iron loss is a loss mechanism similar to the ohmic losses of a resistor. (In a generator, the ohmic losses are called copper losses). The iron loss originates both from eddy currents and hysteresis losses. Eddy currents are currents induced in the iron of the generator while hysteresis loss occurs when magnetic energy is stored and removed from the generator iron. The magnitude of the iron losses depends on the voltage across the windings, and since the voltage across the stator windings in a Y-connection is decreased by a factor of √{square root over (3)}, the iron losses will decrease. Specifically, for a given stator and rotor voltage, the speed range in Y-connection is increased by a factor of √{square root over (3)} compared to the speed range in Δ-connection. For example, if the speed range in Δ-connection is ±36% around synchronous speed, the speed range is extended to ±52% around synchronous speed when connecting the generator in Y-connection. This increased speed and frequency range is derived from analysis of the following relationship between the rotor voltages and the stator voltages:ur=|s|·us·n  (1)where us is the voltage across the stator winding, ur is the voltage across the rotor winding, n is the winding ratio between rotor and stator, and s is the slip.
The output voltage and current from the stator are fed into a medium voltage transformer. The transformer may be located in the top of the turbine or elsewhere. When a transformer is located in the top of a turbine, the transformer can be constructed in at least two ways. The first way is with a primary winding (10 kV) and a secondary winding (690V) and a special tap on the secondary winding (480V). The second way is with a primary winding (10 kV) and a secondary winding (690V) and a tertiary winding (480V). When the medium voltage transformer is not in the top of the turbine, there is still a need for the converter voltage level (480V), and that can also be implemented in several ways, such as either having a transformer with primary winding (690V) and secondary winding (480V) or having a autotransformer with one active winding (690V) but a secondary tap (480V). The medium voltage transformer steps up the voltage to an amount, for example 10 kV at the primary side, required for a power supply, such as a utility grid. The contactor 113, however, is only exemplary and the stator windings can be directly connected to transformer 170 in either Y-connection or Δ-connection. Further, the output from stator 113 can be connected directly to the utility grid or to a separate transformer, instead of transformer 170.
To supply power to/from rotor 112, current induced in rotor 112 is passed through an output filter 140, which is designed to prevent large voltage changes across the generator windings and thereby increase the lifetime of the winding insulation, and then is passed to a back-to-back indirect power converter 150. Power converter 150 includes a converter stage 151, which converts the variable frequency output of generator 110 to a DC voltage, a DC link 152, including an electrolytic capacitance 153, and a converter stage 154, which converts the DC link voltage into a fixed frequency output. The output of converter 154 is fed to a filter 160, which smoothes the current to be supplied and boosts the DC-link voltage. To reduce the voltage ratings of the switches included in converters 151 and 154, the filtered fixed-frequency output is applied to the low-voltage, tertiary windings of transformer 170, for example 480 V.
In accordance with FIG. 1 and assuming ideal components:                               P          m                =                                            P              r                        +                          P              s                                =                                                    sP                s                            +                                                P                  s                                ⁢                                                                   ⁢                where                ⁢                                                                   ⁢                s                                      =                                                            ω                  r                                -                                  ω                  s                                                            ω                s                                                                        (        2        )                                          P          r                =                              sP            m                                1            +            s                                              (        3        )            where Pm is the mechanical input power from the wind, Pr is the power supplied from the rotor circuit, Ps is the power supplied from the stator, and ωr and ωs are the angular frequency of the rotor shaft and the stator field, respectively.
The configuration of FIG. 1, which uses doubly-fed induction generator 110 and indirect power conversion circuit 150, has certain disadvantages. In the turbine of FIG. 1, the switches in the rotor inverter 151 have to be designed to withstand the full load conditions at synchronous speed. At synchronous speed or near synchronous speed, high thermal stress on the switches in the rotor inverter occur because the load on the switches is unequally distributed. As an example, a generator may be running at synchronous speed and delivering a maximum power Pm of 2 MW. At synchronous speed the rotor current Ir is direct current with a frequency of 0 Hz. Ir is calculated as:                               I          r                =                              I            s                                              n              ·              cos                        ⁢                                                   ⁢                          (                              ϕ                n                            )                                                          (        4        )            where n is the winding ratio between rotor and stator, Is is the stator current, and cos(φn) is the nominal displacement angle of the generator when the rotor is short circuited. The maximum stator current Is at synchronous speed is given by:                               I          s                =                              P            max                                              U              s                        ·                          3                                                          (        5        )            where Us is the line-line stator voltage. A typical stator voltage for a wind turbine that produces 2 MW is 690V. Using equation 4 and equation 5, the rotor current is 707 A, assuming a ratio n=2.63 and cos(φn)=0.9. At synchronous speed, the currents in the rotor windings have DC-values, and the current in a specific winding can assume any arbitrary DC-value between zero and 707·√{square root over (2)}. In a worst case scenario, one of the three windings carries a DC-current of 707·√{square root over (2)} while the two windings each carry half (707·√{square root over (2)}/2) of the current but with the opposite sign. (The sum of the rotor currents must at all times equal zero due to the Y-connection of the rotor windings.) At a shaft speed matching synchronous speed, the applied rotor voltage is close to zero. Consequently, the control vectors for the switches in converter 151 are mostly zero-vectors, i.e., either the upper switches of rotor converter 151 are conducting or the lower switches of rotor converter 151 are conducting most of the time. A situation where the upper switches are conducting is shown in FIG. 1C. Hence, each switch in the rotor inverter must be thermally rated to withstand a current of √{square root over (2)}·707 for a given time period while the current ratings at nominal frequency should be 707/√{square root over (2)} meaning a factor 2 in difference.
An additional disadvantage of the FIG. 1 configuration is that capacitance 153 may reduce the efficiency and lifetime of power converter 150. The switches of converter 150 provide only two output levels when coupled to the DC-voltage. As a result, a large filter 160 is needed to reduce harmonic content in the supplied power. Moreover, the harmonic content at the generator side of converter 150 is also high. As a result, a larger filter is required to prevent high voltage changes across the generator windings from causing damage to winding insulation in generator 110.
Matrix converters can also be called either venturini converters or direct frequency converters. Some wind turbines have used matrix converters to eliminate intermediate conversion using a DC link. These wind turbines, however, do not use an actively controlled matrix converter. Prototype wind turbines that have used matrix converters are designed to produce only about 7.5 kW of electricity. A viable design using a matrix converter in a wind turbine to produce electricity at higher power levels has yet to be achieved.
Other documents describe wind turbines and/or direct frequency converters. For example, U.S. Pat. No. 6,137,187 describes a variable speed system with a torque and pitch controller using field oriented control, U.S. Pat. No. 5,949,672 describes a three-phase matrix converter and method for operation thereof, U.S. Pat. No. 5,943,223 describes electric switches for reducing on-state power loss, U.S. Pat. No. 5,909,367 describes a modular AC-AC variable voltage and variable frequency power converter system and control, U.S. Pat. No. 5,892,677 describes an adaptive overlapping communication control of modular AC-AC converter and integration with a device module of multiple AC-AC switches, U.S. Pat. No. 5,852,559 describes power application circuits utilizing bidirectional insulated gate bipolar transistor, U.S. Pat. No. 5,798,631 describes performance optimization controller and control method for doubly-fed machines, U.S. Pat. No. 5,729,118 describes variable speed induction generator-motor with controllable excitation frequency, U.S. Pat. No. 5,669,470 describes a roadway-powered electric vehicle system, U.S. Pat. No. 5,289,041 describes a power converter using a predicted torque set point, U.S. Pat. No. 5,029,064 describes a phase-controlled reversible power conversion with equal duty cycle substantially constant amplitude square wave excitation of the power transformer, U.S. Pat. No. 4,648,022 describes a matrix converter control system, U.S. Pat. No. 4,468,725 describes a direct AC converter for converting a balanced AC polyphase input to an output voltage, U.S. Pat. No. 4,439,823 describes converting multiphase power from one frequency to another using current waveforms, U.S. Pat. No. 4,352,155 describes a variable speed constant frequency power converter with two modes of operation, and U.S. Pat. No. 3,832,625 describes an electrical power generating arrangement and method using an induction generator. Each of these U.S. patents are incorporated by reference herein in its entirety.